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Analysis of Optical Glass Lens Molding Technology

    Optical glass lens molding technology is a high-precision optical component manufacturing process. It involves placing softened glass into a high-precision mold and directly molding it into an optical part that meets usage requirements in a single step under conditions of heating, pressing, and an oxygen-free environment. Since its successful development in the mid-1980s, this technology has a history of over a decade and has become one of the most advanced optical component manufacturing methods internationally. It has entered the practical production stage in many countries. The widespread adoption and application of this technology represent a significant revolution in the processing of optical glass components within the optics industry. Because this technology enables the direct molding of precision aspheric optical components, it has ushered in an era where optical instruments can widely utilize aspheric glass optical elements. Consequently, it has brought new changes and developments to the optical system design of optoelectronic instruments. Not only has it reduced the size and weight of optical instruments, saved materials, decreased the workload for optical component coating and assembly, and lowered costs, but it has also improved the performance of optical instruments and enhanced the quality of optical imaging.

Manufacturing optical components using the glass molding method offers the following advantages:

1. 1.Eliminates the need for traditional processes like rough grinding, fine grinding, polishing, edging, and centering, yet achieves high dimensional accuracy, surface figure accuracy, and surface roughness.

2. 2.Saves a significant amount of production equipment, tooling/auxiliary materials, factory floor space, and skilled workers, enabling high productivity even in a small workshop.

3. 3.Facilitates economical mass production of precision aspheric optical components.

4. 4.Ensures dimensional accuracy and repeatability of molded optical parts by precisely controlling process parameters like temperature and pressure during molding.

5. 5.Enables molding of small aspheric lens arrays.

6. 6.Allows optical components and mounting reference features to be formed as a single integrated piece.

Currently, mass-produced molded aspheric optical components have diameters ranging from 2 to 50 mm with a tolerance of ±0.01 mm; thicknesses from 0.4 to 25 mm with a tolerance of ±0.01 mm; curvature radii down to 5 mm; surface figure accuracy of 1.5λ; surface roughness conforming to US Military Standard 80-50; refractive index controllable to ±5×10⁻⁴; refractive index homogeneity controllable to <5×10⁻⁶; and birefringence less than 0.01λ/cm.

World-renowned companies and manufacturers that have mastered this advanced glass optical component manufacturing technology include Kodak and Corning (USA); Ohara, Hoya, Olympus, and Panasonic (Japan); Zeiss (Germany); and Philips (Netherlands).

Glass optical component molding technology is a comprehensive discipline requiring the design of specialized molding machines, the use of high-quality molds, and the selection of appropriate process parameters. The molding method, glass type and blanks, mold material and fabrication are all key technologies in glass molding.

1. Molding Methods

The primary reason glass can be precision molded stems from the development of mold materials that do not adhere to softened glass.

The original glass lens molding method involved pouring molten optical glass blanks into molds maintained at a temperature about 50°C above the glass transition temperature (Tg) for pressing. This method was prone to glass sticking to the mold surfaces, and the products often suffered from bubbles and cold-mold marks (wrinkles), making it difficult to achieve the desired shape and surface figure accuracy. Later, a method emerged using precisely machined molds made from special materials. The glass and mold are heated together in an oxygen-free atmosphere to near the glass softening point. When both reach approximately the same temperature, pressure is applied via the mold. While maintaining this pressure, the mold is cooled down below the glass transition temperature (Tg) (Glass viscosity at softening point ≈ 10⁷.⁶ Poise; viscosity at transition point ≈ 10¹³.⁴ Poise). This method of pressing glass and mold together isothermally is called isothermal pressing, which relatively easily achieves high precision by accurately replicating the mold surface shape. The drawback of this method for manufacturing glass optical components is the long time required for heating and cooling, resulting in slow production speeds. To address this, effective improvements were made, such as using multiple molds within a single press to increase efficiency. However, the high cost of aspheric molds makes using multiple molds prohibitively expensive. To tackle this, research focused on developing non-isothermal pressing methods closer to the original blank molding conditions, aiming to increase the production speed per mold and extend mold lifespan. Additionally, research is ongoing into methods for directly precision molding glass flowing from a melting furnace.

2. Glass Types and Blanks

The glass blank is directly related to the quality of the molded product. In principle, most optical glasses can be used for molding. However, glasses with high softening points require high molding temperatures, which can cause slight reactions with the mold, drastically shortening mold lifespan. Therefore, from the perspective of easier mold material selection and longer mold life, glasses suitable for molding at lower temperatures (around 600°C) needed to be developed. These low-temperature molding glasses must also meet requirements for cost-effective blank production and be free of environmentally hazardous substances (e.g., PbO, As₂O₃). Blanks used for molding have specific requirements:

① The blank surface must be extremely smooth and clean before pressing;

② It must have a suitable geometric shape;

③ It must have the required volume. Blanks are typically spherical, puck-shaped, or spherical-lens shaped, produced by cold grinding or hot pressing.

3. Mold Materials and Machining

Mold materials must possess the following characteristics:

① Surface free of defects, capable of being polished into a smooth, pore-free optical surface;

② High oxidation resistance at high temperatures, maintaining structural integrity, stable surface quality, surface figure accuracy, and finish; ③ Non-reactive with glass, no adhesion, good release properties;

④ High hardness and strength at elevated temperatures.

Numerous patents exist for developing mold materials. Representative materials include: Super hard alloy substrates coated with precious metal alloys and titanium nitride (TiN) films; Silicon carbide (SiC) or super hard alloy substrates coated with hard carbon, diamond-like carbon (DLC), or other carbon-based films; and Cr₂O-ZrO₂-TiO₂ based new ceramics.

Materials for glass lens molding molds are generally hard and brittle. To precisely machine these materials into molds, high-rigidity, ultra-precision CNC machines with resolution below 0.01 μm are required, using diamond grinding wheels. Grinding achieves the desired shape accuracy, but subsequent fine polishing is necessary to achieve an optical surface finish. For high-precision aspheric machining, aspheric surface measurement and evaluation techniques are crucial. Machining molds for micro-lenses demands even stricter requirements, necessitating higher precision and minimized grinding marks.

4. Applications of Glass Molding Technology

Currently, optical glass lens molding technology is used for mass production of precision spherical and aspheric lenses. Besides routinely producing lenses around 15mm in diameter, it can also produce large-diameter lenses up to 50mm, micro-lens arrays, etc. Micro-lens arrays with individual lens diameters as small as 100μm are now achievable.

1. 1.Manufacturing spherical and aspheric optical components for military and civilian optical instruments, such as various lenses, prisms, and filters.

2. 2.Manufacturing aspheric lenses for fiber optic couplers used in optical communication.

3. 3.Manufacturing aspheric condenser lenses for optical discs. A single molded aspheric lens can replace three spherical lenses within an optical disc reader's optical head. Due to the high precision of molded aspheric lenses, they not only control and correct axial aberrations at large numerical apertures (NA) but also reduce the weight of the original optical head and lower costs by 30-50%.

4. 4.Manufacturing aspheric lenses for camera viewfinders, movie projectors, and camera lenses.